1. Field of the Invention
This invention relates to a dielectric ceramic composition of matter and more particularly to a dielectric ceramic composition of matter useful as a material for a resonator and a circuit substrate in the microwave frequency band.
2. Prior art
In recent years, dielectric ceramics have found wide application in the microwave frequency band such as in the development of a microwave circuit into use of integrated circuit along with the practical use of an automobile telephone, cordless telephone, personal radio set, and satellite broadcast receiver; in the increased range of use of Gunn oscillators; and in use for gallium aresnide field-effect transistor (GaAs FET) driven oscillators and the like.
Such a dielectric ceramic for microwave use is chiefly used in a resonator, and characteristic features demanded of the resonator are included under the following three characteristics that (1) the dielectric ceramic should have the highest possible dielectric constant because, in the dielectric, the wave length is shortened to 1/.sqroot..epsilon..tau. (wherein .epsilon..tau. represents a dielectric constant) and because a ceramic larger in dielectric constant facilitates so much the greater reduction in size if the ceramic is the same in resonance frequency; (2) the ceramic should be small in dielectric loss with respect to high frequency; and (3) it should be small in its resonance frequency change according to temperature, namely, small and stable in the temperature dependency of its dielectric constant. And if a resonator is on the order of 1 GHz considered to be in a relatively low-frequency band which is used for an automobile telephone, personal radio set, cordless telephone, and the like, even in the microwave frequency band, the wavelength to be applied to the low-frequency band is considerably increased in wavelength, and accordingly miniaturization of the resonator requires considerably high dielectric constant.
Heretofore, it was known that the dielectric ceramics of the type described above are, for example, a BaO-TiO.sub.2 based material; BaO-REO-TiO.sub.2 based material (wherein REO represents an oxide of rare-earth element and the same shall apply hereinafter); and (BaSrCa) (ZrTi)O.sub.3 based material and so on.
But while BaO-TiO.sub.2 based material and (BaSrCa) (ZrTi)O.sub.3 based material each have an excellent low dielectric loss in a microwave frequency band of 4.about.10 GHz, they are as low in dielectric constant as 29.about.40, and particularly miniaturization of the resonator in a frequency band of the order of 1 GHz poses a problem in point of practical use. An increase in dielectric constant tends to extremely deteriorate the temperature characteristic or dielectric loss of resonance frequency.
On the other hand, systematic experiment reports on the BaO-REO-TiO.sub.2 based material [R. L. Bolton "Temperature Compensating Ceramic Capacitors in the System Baria-Rare Earth Oxide-Titania" Ph.D. Thesis, University of Illinois-Urbana, 1968 and Drago Kolar et al., Ber Deutsch Keram. Ges., 55. 346.about.348 (1978)] are available. The reports are all based on the measurement made at 1 MHz and no application of the material in a microwave frequency band of the order of 1 GHz was contemplated. When the material is used in the form of a dielectric resonator, the dielectric material is as high in dielectric constant as 70.about.80 and also small in dielectric loss, but is so very high in the temperature coefficient of dielectric constant as N 100.about.N 150 ppm/.degree.C. (wherein N represents negative) on the N side, and the temperature coefficient (+.tau.) of resonance frequency becomes high on the P (positive) side. For example, when the resonator is used in a band-pass filter as of a personal radio set, a transmitting and receiving band deflects from a specified frequency band because of a temperature change and enters the adjacent frequency band, with the result that it becomes difficult to send and receive signals, posing a problem in point of practical use. Also, even if an amount of Nd.sub.2 O.sub.3 is for example increased to modify (.tau.f) to NPO (in the direction of 0), the dielectric material is subject to reduced dielectric constant and increased dielectric loss. Accordingly, the dielectric ceramic composition of matter for use in microwaves having the above-mentioned three characteristics has not yet been developed.